Telephone line coupler

ABSTRACT

The telephone line coupler circuit has a single transmit opto-coupler whose output includes a DC bias component connected to a gate of a line seize switch for connecting a DC line seize load across the ring and tip contacts of the telephone line. The line seize switch is saturated by the transmit opto-coupler bias output and the AC component of the transmit opto-coupler output is sent over the telephone lines. The receive opto-coupler is used both for receiving communications signal and for detecting the ring signal. The band pass filter connected to the output of the receive opto-coupler may be switched to pass a ring signal frequency band or a communications frequency band. The circuit operates using two opto-coupler devices while conventional circuits require four opto-coupler devices.

FIELD OF THE INVENTION

The present invention relates to a telephone line coupler circuit forconnecting telephone subscriber equipment to a telephone line.

BACKGROUND OF THE INVENTION

A telephone line coupler circuit is found in most every type ofelectronic equipment connected to a telephone line such as modems andfax machines. In order to protect the electronic equipment from surgeson the telephone line and side to prevent different ground voltages fromcausing erroneous operation in the subscriber equipment, transformers oropto-couplers are used in the coupler circuit to connect the subscriberelectronic equipment for the telephone line.

Coupler circuits which use opto-couplers are known in the art. In U.S.Pat. No. 4,727,535 to Brandt, a coupler circuit is described in which asingle opto-coupler is used for relaying the analog AC transmit signaland another opto-coupler device is used for relaying the received ACsignal. The telephone line connect and disconnect circuit (e.g. a linerelay) is not disclosed. In U.S. Pat. No. 4,203,006 to Mascia, oneopto-coupler is used for relaying a ringing signal to a modem, a secondopto-coupler is used in relaying a line seize signal from the modem tothe telephone line access coupler and a transformer is used in place ofa pair of opto-couplers for relaying the received and transmitted ACsignals from the coupler to the modem.

In the known prior art coupler circuits using opto-couplers, the basicfunctions of relaying the ring signal, relaying the communicationsreceive signal, relaying the line seize signal and relaying the ACtransmit signal all require separate opto-coupler devices. In the casethat a transformer used, the bi-directional nature of the transformerallows for single device to be used for relaying the received and thetransmitted communication signal. In a standard telephone line couplercircuit, the cost of the opto-coupler devices is a substantial portionof the component cost for the circuit.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide atelephone line coupler circuit for coupling a telephone line to asubscriber electronic device with isolation between the telephone lineand the subscriber device in which the number of opto-couplers isreduced.

In accordance with the first aspect of the present invention, there isprovided a telephone line coupler circuit for connecting telephonesubscriber equipment to a telephone line, the circuit comprising: ringand tip connector means for connecting to telephone line ring and tipcontacts and providing ring and tip signal outputs; a high impedance DCload and a gated line switch connected in series between the ring andtip signal outputs for controllably conducting an “off-hook” currentbetween the ring and tip signal outputs; a transmit opto-coupler meanshaving a transmit signal input and output; means connecting the outputterminal to the tip output and to a gate input of the gated line switch;means for controllably providing a low level DC bias signal to thetransmit signal input and generating sufficient current on the output tosubstantially saturate the gated line switch and seize the line; meansfor providing an outgoing AC signal to the transmit signal input; and ACsignal receive means connected to the ring and tip outputs for detectingan incoming AC signal and producing an incoming AC signal output.

The invention also provides a telephone line coupler circuit forconnecting telephone subscriber equipment to a telephone line, thecircuit comprising: ring and tip connector means for connecting totelephone line ring and tip contacts and providing ring and tip signaloutputs; a high impedance DC load and a line switch connected in seriesbetween the ring and tip signal outputs for controllably conducting an“off-hook” current between the ring and tip signal outputs; signaltransmit means having a transmit signal input and being connected to thering and tip outputs for transmitting AC signal; AC communicationssignal receive means connected to the ring and tip outputs for detectingan incoming AC communications signal and producing an incoming ACcommunications signal output; and AC ring signal detect means connectedto the ring and tip outputs for detecting a telephone ring signal on thetelephone line and generating a ringing output signal; wherein: the ACcommunications signal receive means and the AC ring signal detect meansshare a common receive opto-coupler device.

Preferably, the AC communication signal receive means and the AC ringsignal detect means comprise a single band pass amplifier circuit whichis switchable between two frequency bands, that is a first frequencyband for the telephone ring signal and a second frequency band forreceived communication signals. Also preferably, the band pass filteramplifier is switched between the ring frequency band to thecommunications frequency band by the line seize signal connected to themeans for controllably providing a load level DC bias signal to thetransmit signal input of the transmit opto-coupler means.

Accordingly, the invention also provides a telephone line couplercircuit for connecting telephone subscriber equipment to a telephoneline, the circuit comprising: ring and tip connector means forconnecting to telephone line ring and tip contacts and providing ringand tip signal outputs; a high impedance DC load and a gated line switchconnected in series between the ring and tip signal outputs forcontrollably conducting an “off-hook” current between the ring and tipsignal outputs; a transmit opto-coupler means having a transmit signalinput and output; means connecting the output terminal to the tip outputand to a gate input of the gated line switch; means for controllablyproviding a low level DC bias signal to the transmit signal input andgenerating sufficient current on the output to substantially saturatethe gated line switch and seize the line; means for providing anoutgoing AC signal to the transmit signal input; AC communicationssignal receive means connected to the ring and tip outputs for detectingan incoming AC communications signal and producing an incoming ACcommunications signal output; and AC ring signal detect means connectedto the ring and tip outputs for detecting a telephone ring signal on thetelephone line and generating a ringing output signal; wherein: the ACcommunications signal receive means and the AC ring signal detect meansshare a common receive opto-coupler device.

The invention also provides a method of isolating and connecting atransmit signal generated by subscriber electronic equipment to atelephone line.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood by way of the following detaileddescription of a preferred embodiment with reference to the appendeddrawing in which:

FIG. 1 is a block diagram of the telephone line coupler circuitaccording to the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the preferred embodiment as illustrated in FIG. 1, the telephone linecoupler circuit according to the invention comprises a telephone linering/tip connector providing ring and tip outputs 10 and 11respectively. In the case that the polarity may be reversed by thetelephone company, a diode bridge 12 is provided for providing thecorrect polarity to the rest of the circuit. A receive opto-coupler 16is connected to the positive ring output of the diode bridge 12 througha capacitor and line seize switch 15 to the negative tip output ofbridge 12. A DC line seize load 14 and a line seize switch 15 areconnected in series between the ring and tip outputs of bridge 12. Inorder to seize the telephone line, a small DC current must pass throughthe coupler circuit in order for the telephone company central officeequipment to consider the line to be in use.

A transmit opto-coupler 20 has an input consisting of a DC bias signalfrom a DC bias source 22 and an AC signal component coming from transmitamplifier 24. A comparator circuit is used to block the AC signal in theabsence of the DC bias signal and to allow the DC bias signal to passthrough in the absence of the AC signal. When the line seize signal 23is energized, the DC bias voltage from 22 causes the transmitopto-coupler to output a base DC level which saturates the line seizeswitch 15 to provide the minimum required DC current through load 14 toseize the line. The DC bias level is chosen to provide a sufficientoutput to saturate line seize switch even when the AC component fromtransmit amplifier 24 is superimposed. The AC output from opto-coupler20 is also fed through a resistance directly to the tip output of bridge12. For pulse dialing, a separate pulse dialing input is provided whichis ORed with line seize signal 23.

When the circuit is in the “on-hook” state, line seize signal 23 is lowand the band pass filter amplifier 18 is set to amplify AC signals in afrequency range of the telephone ring signal. Current from outputs 10and 11 corresponding to a ring signal pass through ring signal filter 13across the receive opto-coupler 16. Return current passes through diode17. The received communication/ring signal output from amplifier 18produces an AC output corresponding to the ring signal appearing onoutputs 10 and 11. The subscriber device connected to the output ofamplifier 18 detects the presence of the ring signal and upon decidingto answer, places an output on line seize signal line 23 resulting inamplifier 18 switching to filtering and amplifying frequencies in thecommunication band and resulting in DC bias source 22 providing a DCbias signal to transmit opto-coupler 20 which causes the line seizeswitch 15 to be saturated and the line to be seized by passing therequired DC current through load 14. Since the receive opto-coupler 16is connected to the transmit opto-coupler 20, a minimum draw of currentto place the light-emitting diode of receive opto-coupler 16 in anoperational range is achieved. As an AC signal comes into receiveopto-coupler 16, a faithful isolated AC output is generated.

As can be appreciated, the line coupler circuit according to theinvention requires only two opto-coupler devices for the purposes ofrelaying ring signal, communication signal, transmission and seizing thetelephone line. While in the preferred embodiment, the band pass filteramplifier 18 is shown as a single block circuit having a single output,it is of course possible to provide two separate filter/amplifiercircuits connected to the output of opto-coupler 16 without requiring aconnection to line seize signal line 23 to switch between the twofilters.

1. A telephone line coupler circuit for connecting telephone subscriberequipment to a telephone line, the circuit comprising: ring and tipconnector means for connecting to telephone line ring and tip contactsand providing ring and tip signal outputs; a high impedance DC load anda gated line switch connected in series between said ring and tip signaloutputs for controllably conducting an “off-hook” current between saidring and tip signal outputs; a transmit opto-coupler means having atransmit signal input and output; means for connecting said transmitsignal output to said tip output and to a gate input of said gated lineswitch; means for controllably providing a low level DC bias signal tosaid transmit signal input and generating sufficient current on saidtransmit signal output to substantially saturate said gated line switchand seize said telephone line; means for providing an outgoing AC signalto said transmit signal input; and AC signal receive means connected tosaid ring and tip outputs for detecting an incoming AC signal andproducing an incoming AC signal output.
 2. A telephone line couplercircuit for connecting telephone subscriber equipment to a telephoneline, the circuit comprising: ring and tip connector means forconnecting to telephone line ring and tip contacts and providing ringand tip signal outputs; a high impedance DC load and a gated line switchconnected in series between said ring and tip signal outputs forcontrollably conducting an “off-hook” current between said ring and tipsignal outputs; a transmit opto-coupler means having a transmit signalinput and output; means for connecting said transmit signal output tosaid tip output and to a gate input of said gated line switch; means forcontrollably providing a low level DC bias signal to said transmitsignal input and generating sufficient current on said transmit signaloutput to substantially saturate said gated line switch and seize saidtelephone line; means for providing an outgoing AC signal to saidtransmit signal input; AC communications signal receive means connectedto said ring and tip outputs for detecting an incoming AC communicationssignal and producing an incoming AC communications signal output; and ACring signal detect means connected to said ring and tip outputs fordetecting a telephone ring signal on said telephone line and generatinga ringing output signal; wherein: said AC communications signal receivemeans and said AC ring signal detect means share a common receiveopto-coupler device.
 3. A telephone line coupler circuit for connectingtelephone subscriber equipment to a telephone line, the circuitcomprising: ring and tip connector means for connecting to telephoneline ring and tip contacts and providing ring and tip signal outputs; ahigh impedance DC load and a line switch connected in series betweensaid ring and tip signal outputs for controllably conducting an“off-hook” current between said ring and tip signal outputs; signaltransmit means having a transmit signal input and being connected tosaid ring and tip outputs for transmitting AC signal; AC communicationssignal receive means connected to said ring and tip outputs fordetecting an incoming AC communications signal and producing an incomingAC communications signal output; and AC ring signal detect meansconnected to said ring and tip outputs for detecting a telephone ringsignal on said telephone line and generating a ringing output signal;wherein said AC communication signal receive means and said AC ringsignal detect means share a common receive opto-coupler device and acommon band pass filter circuit switchable between a frequency band ofsaid ring signal and a frequency band of communications signal receivedover said telephone line, a switching of said filter circuit being inresponse to a control signal for said line switch.
 4. A telephone linecoupler circuit for connecting telephone subscriber equipment to atelephone line, the circuit comprising: ring and tip connector means forconnecting to telephone line ring and tip contacts and providing ringand ti-p tip signal outputs; a high impedance DC load and a gated lineswitch connected in series between said ring and tip signal outputs forcontrollably conducting an “off-hook” current between said ring and tipsignal outputs; a transmit opto-coupler means having a transmit signalinput and output; means connecting said transmit signal output to saidtip output and to a gate input of said gated line switch; means forcontrollably providing a low level DC bias signal to said transmitsignal input and generating sufficient current on said transmit signaloutput to substantially saturate said gated line switch and seize saidline; means for providing an outgoing AC signal to said transmit signalinput; AC communications signal receive means connected to said ring andtip outputs for detecting an incoming AC communications signal andproducing an incoming AC communications signal output; and AC ringsignal detect means connected to said ring and tip outputs for detectinga telephone ring signal on said telephone line and generating a ringingoutput signal; wherein said AC communications signal receive means andsaid AC ring signal detect means share a common receive opto-couplerdevice and include a shared band pass filter circuit switchable betweena ring signal frequency band and a communication signal frequency band,said filter circuit being switched by a line seize signal fed to saidlow level DC bias signal providing means.
 5. A telephone line couplercircuit for connecting telephone subscriber equipment to a telephoneline, the circuit comprising: a ring and tip connector circuitryconnecting to telephone line ring and tip contacts and providing ringand tip signal outputs; a high impedance DC load and a gated line switchconnected in series between said ring and tip signal outputs forcontrollably conducting an “off-hook” current between said ring and tipsignal outputs; a receive opto-coupler having a receive signal input ona line side and a receive signal output on an equipment side; a transmitopto-coupler having a transmit signal input on said equipment side and atransmit signal output on said line side, said receive opto-coupler andsaid transmit opto-coupler being connected in series on said line side;said transmit signal output being connected to said tip output and to agate input of said gated line switch; a line side transmit signal and DCbias signal combining circuitry receiving a transmit communicationssignal and a DC bias signal and outputting a combined signal to saidtransmit signal input; and a DC bias voltage generator responsive to acontrol signal to provide a low level DC signal as said DC bias signal,wherein sufficient current is generated on said transmit signal outputto substantially saturate said gated line switch and seize saidtelephone line.
 6. The circuit as claimed in claim 5, furthercomprising: AC ring signal detection circuitry connected to saidtelephone line ring and tip contacts and outputing a ring signal to saidreceive signal input.
 7. The circuit as claimed in claim 5, wherein saidring and tip connector circuitry comprises a diode bridge.
 8. Thecircuit as claimed in claim 6, further comprising a common band passfilter circuit connected to said receive signal output and switchablebetween a frequency band of said ring signal and a frequency band of acommunications signal received over said telephone line, a switching ofsaid filter circuit being in response to said control signal.
 9. Thecircuit as claimed in claim 6, wherein said ring and tip connectorcircuitry comprises a diode bridge, said AC ring signal detectioncircuitry being connected to said ring and tip signal contacts beforesaid diode bridge.
 10. The circuit as claimed in claim 8, wherein saidring and tip connector circuitry comprises a diode bridge, said AC ringsignal detection circuitry being connected to said ring and tip signalcontacts before said diode bridge.
 11. A method of isolating andconnecting a transmit signal generated by subscriber electronicequipment to a telephone line, the method comprising: providing atransmit opto-coupler; adding a DC bias to the electronic equipmenttransmit signal coming from the electronic equipment to obtain acombined signal and feeding the combined signal to the transmitopto-coupler, said DC bias being sufficient to generate a low level DCoutput on a line side of said transmit opto-coupler; using said lowlevel DC output to trigger a line seize circuit connected to saidtelephone line to draw a minimum current required by a central office toseize the telephone line; transmitting an isolated copy of said transmitsignal output from said transmit opto-coupler on said telephone line,using circuitry separate from the line seize circuitry; providing areceive opto-coupler; and connecting said transmit opto-coupler and saidreceive opto-coupler together in series on a telephone line side.
 12. Amethod of isolating and connecting a ring signal and a communicationsreceive signal on a telephone line to subscriber electronic equipment,the method comprising: providing a receive opto-coupler; connecting saidreceive opto-coupler using first circuit elements to the telephone lineto detect and isolate an incoming AC communications signal; connectingsaid receive opto-coupler using second circuit elements to the telephoneline to detect and isolate a ring signal; providing a transmitopto-coupler; connecting said transmit opto-coupler and said receiveopto-coupler together in series on a telephone line side; detecting thering signal at an output of the receive opto-coupler in the subscriberelectronic equipment when the subscriber electronic equipment is in anon-hook state; and detecting the incoming AC communications signal atthe output of the receive opto-coupler in the subscriber electronicequipment when the subscriber electronic equipment is in an off-hookstate; wherein said steps of detecting comprise using differentfiltering characteristics on said output of the receive opto-couplerdepending on the on-hook/off-hook state.